COMPARISON OF PLATINUM AXI <:\^ I IIKl;\! )\ll I I |> 07 



XXVI. DESCRIPTION OF THE GAS THERMOMETER. 



The gas thermometer we employed for our researches is similar to the instrument 

 previously described by one of us* and is shown in fig. 12. 



It is a constant-volume thermometer arranged so as to permit the determination 

 of tin- total pressure of the gas contained in the thermometric reservoir by a single 

 ,-ftilint/. The simplification thus introduced into the measurements permits rapid 

 observations without sensibly diminishing the precision, which is limited more by the 

 eoii.litions ,,f uniformity of the baths and other heating arrangements employed than 

 hv reading errors. 



The apparatus constituting the gas thermometer is installed on a foundation of 

 Concrete about 1 cubic metre in volume abutting against one of the massive walls of 

 the lal .oratory. On this foundation on the left hand rests a rectangular stone pillar, 

 with slate top, carrying the manometric apparatus, while the heating baths on the 

 i -ight are supported on the same foundation at floor level, this however not being 

 sho\\n in fig. 12. The manometric apparatus is protected from heating by a large 

 pa|>er screen which extends upward right to the ceiling, and which is pierced with 

 the holes necessary to admit the passage of the various connecting portions. 



(a) Thermometric Reservoir. 



For the first part of the experiments we employed a cylindrical reservoir of 

 hard glass drawn from a tube 36 millims. external diameter and I '5 millims. thick. 



This tube, closed at one extremity, was fused at the other to a capillary tube 

 of the same glass, having a bore of 0'53 millim. and 90 centims. long. Fig. 12 

 shows the thermometric reservoir mounted for the experiments, its axis being in the 

 vertical position, which we find the most convenient for the introduction of the 

 reservoir into the various baths employed. The outer l>end of the tule carrying the 

 reservoir is supported by a light frame from the iron girder carrying the manometer, 

 and slides vertically along this through a considerable distance. 



The porcelain reservoir employed for work at high temperatures was obtained from 

 the Royal Porcelain Factory at Berlin. It is cylindrical in shape and is 36 millims. 

 external diameter and 20 centims. long. The reservoir has a neck 28 centims. in 

 length and 11 millims. exterior diameter. It is pierced with a hole of 'J millims. 

 diameter, into which just passes the platinum capillary uniting the reservoir to the 

 manometer. The joining of the platinum tulx- to the neck of the manometer is a 

 matter of some difficulty, since it is of great importance that this joint should be 

 absolutely gas-tight. We finally adopted the following disposition which has answered 

 well. On the platinum tube which enters to a length of 1 1 centima into the neck 

 of the reservoir, a brass washer is soldered which fits exactly to the Hut end of the 



* ' Trav. et M4m. du Bureau International,' vol. 6, p. 28. 

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